A ferrule used in optical communication is an important component of an optical-fiber connector. The ferrule is a cylindrical component having a through hole for insertion of an optical fiber made of silica glass. Connection of optical fibers is carried out by inserting and fixing optical fibers in the through holes of ferrules, respectively, polishing their ends, and thereafter, inserting the ferrules in an outer tube called a sleeve to face-to-face contact with each other. Therefore, the ferrule must have an excellent dimensional accuracy and is further required to have various material properties such as high mechanical strength, high abrasion resistance against friction upon insertion and removal into and out of the sleeve, polishability which is close to that of silica glass so that a polished amount of the ferrule does not greatly differ from that of the optical fiber during polishing the end, and a thermal expansion coefficient close to that of silica glass. In order to organize an optical communication network well, it is an urgent demand to provide a ferrule which satisfies the above-mentioned material properties and which can be manufactured at a low cost.
At present, there have been used ferrules made of ceramics such as zirconia and alumina and of amorphous glass. Among them, the ferrule made of ceramics is excellent in mechanical strength and abrasion resistance but has the following disadvantages. Specifically, it is considerably low in a rate to be polished as compared with the fiber of silica, so that it is necessary to use a special polishing method. It is so high in thermal expansion coefficient as compared with the optical fiber that an initial connection loss is readily deteriorated by the changes in dimension of both of them following a temperature variation. Furthermore, it is so low in formability and machinability to degrade the production efficiency, resulting in a high cost.
On the other hand, the ferrule made of amorphous glass is advantageous in that the polished rate is very close to that of the silica fiber to require no special polishing method, resulting in a reduced polishing cost, and that the formability and the machinability are excellent to enable the production at a low cost. However, it is disadvantageous in that the mechanical strength and the abrasion resistance are insufficient.
In addition, a proposal is made of a ferrule of crystallized glass. For example, JP-B S63-500684 discloses an art in which the crystallized glass of Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 is used as a ferrule material. In the art, it is possible to obtain the ferrule which has the thermal expansion coefficient close to that of the silica fiber, but there is neither consideration about the abrasion resistance, the polishability, and the machinability, nor knowledge is given about those properties. Furthermore, JP-A H1-288803 discloses a connection component made of the crystallized glass of P.sub.2 O.sub.5 --CaO. In the art, it is possible to obtain the ferrule which is excellent in machinability, but there is no consideration about the abrasion resistance and the mechanical strength. In addition, it has cost-rising factors such as the use of a laser to form the through hole, and others.
As described above, there is no such ferrule in the present status that satisfies all of the requirements for use in the optical-fiber connector.
In the meanwhile, the crystallized glass is generally higher in strength than the amorphous glass because of an energy absorption effect given by bending and branching of cracks at the interface between precipitated crystals and a glass matrix. Further, crystallization makes it possible to obtain a material improved in abrasion resistance and low in thermal expansion coefficient. In addition, the crystallized glass advantageously has a formability suitable for mass production, like the amorphous glass.
Accordingly, the use of the crystallized glass has a possibility of providing a ferrule at a low manufacturing cost but with excellent characteristics unexpected in conventional ferrule materials.
However, the above-mentioned excellent features of the crystallized glass widely vary depending upon the kind, the size, and the amount of the precipitated crystals. In an unsuitably crystallized state, the characteristics necessary to the ferrule can not be realized.
In view of the above, it is an object of this invention to provide a ferrule for an optical-fiber connector, which is excellent in mechanical strength and abrasion resistance.
It is another object of this invention to provide a ferrule for an optical-fiber connector, which has a thermal expansion coefficient and a polishability close to those of an optical fiber.
It is still another object of this invention to provide a ferrule for an optical-fiber connector, which has a dimensional accuracy equivalent to that of a zirconia ferrule used for connecting a single mode optical fiber.
It is a further object of this invention to provide a ferrule for an optical-fiber connector, which can be manufactured at a low cost as compared with a zirconia ferrule.